Metal silicide photovoltaic cells are commonly used as photodetector elements in infrared (IR) imaging arrays. In particular, platinum silicide (PtSi) technology is well developed, and reliable PtSi focal plane arrays with uniform response can be manufactured with good yield and reproducibility. One drawback of PtSi arrays, however, is their low sensitivity. A typical PtSi layer will capture only 10% or less of the impinging photons per pass at a wavelength of 4 .mu.m, for example. A reflective backing is commonly used on PtSi arrays to provide a double pass; however, the absorbency of a conventional cell is still less than 20%. In addition, because of the need for good resolution, individual cells of an IR detector array are severely constrained in size. Consequently, the output of individual cells is so low as to be seriously affected by electronic noise.
Prior art in the field includes U.S. Pat. No. 4,496,964 to Tsubouchi et al., which shows p-type polysilicon deposited on a PtSi layer to form a phototransistor; U.S. Pat. No. 4,524,374 to Denda et al. which concerns a single-layer PtSi phototransistor; U.S. Pat. No. 4,544,939 to Kosonocky et al. which deals with extending the wavelength response of a PtSi photodetector by interposing a high-impurity implant between the PtSi layer and the substrate; and U.S. Pat. No. 4,586,069 to Koniger et al which also concerns the interposition of a highly doped layer in a PtSi-silicon photodiode to provide improved sensitivity at specific wavelengths.